Teh harsh vocals on that song are by Shagrath of Dimmu Borgir. I wouldn't get too wrapped up in the whole "Satanist band" thing there, though, because I know for a fact that they've performed the song live with a Christian doing the harsh vocals (Matt Knowles of Royal Anguish). RA's bassist, Sean Tibbets is, if I remember properly, a former member of Kamelot, and he's done the "fill in" thing for Kamelot on tour several times as well. Chances are, they just liked they way Shagrath's vocals sound, and invited him to guest on the track because his sound fit the part.
As far as the whole "are they acceptable" thing? there IOS no easy or singular "correct" answer to that. You'll get as many different answers to that question as there are metal loving Christians out there....the best thing to do is examine the lyrics and the message (if there IS one, other than fictional storytelling), and decide for yourself thru prayer and introspection how their impact upon you meshes with Scripture.
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Obama is no more the Anti-Christ than George Bush is Sauron, sending his armies into Iraq searching for the One Ring.....
Dwarven gravitational theory states that enough ale will make anything fall down.
Madness does not always howl. Sometimes, it is the quiet voice at the end of the day saying, "Hey, is there room in your head for one more?"